This invention relates to a process for preparing partially hydrogenated fluorinated hydrocarbons, in particular it relates to the preparation of 1,2-difluoroethane and of 1,1,2-trifluoroethane by catalytic hydrogenation of 1,2-dichlorodifluoroethylene and 1-chlorotrifluoroethylene, respectively.
1,2-difluoroethane CH.sub.2 F--CH.sub.2 F is a well-known fluorinated hydrocarbon which is utilizable, in admixture with other hydrocarbons, as a fluid for the Rankine cycle, as a cooling medium (U.S. Pat. No. 4,055,049) or as a starting material for the preparation of fully substituted brominated derivatives.
1,2-difluoroethane and 1,1,2-trifluoroethane can be used also as components of propellant mixtures for aerosols.
In fact, although the assumption that a possible accumulation of fluorinated hydrocarbons may lead to a degradation of the stratospheric ozone is still to be proved, conversely it seems probable that the hydrogen-containing fluorocarbons do not raise any problems in this respect.
The prior art does not describe any specific method of preparing 1,2-difluoroethane which can be easily practiced on an industrial scale, or which is capable of providing this product with a high yield and a high selectivity. Conversely, the prior art describes general methods for the treatment of variously substituted ethane, from which it is possible to obtain a mixture of products in which also 1,2-difluoroethane may be present as a by-product.
These general methods include the direct fluorination of CH.sub.3 --CH.sub.2 F with F.sub.2 (Cadmau P., Kirk A. W.; Trotman--Dickenson A. F. J. Chem. Soc., Faraday Trans. 1), the electrochemical fluorination of ethane at 100.degree. C. (Fox H. M., Ruehlen F. N. Childs, W. V.; J. Electochem. Soc. 1971, 118(7), 1246-9) and the fluorination of ethane with potassium tetrafluorocolbaltate (KCoF.sub.4) and CoF.sub.3 (Burdon J.; Knights J.; Parson I.; Tatlow J. - Tetrahedron 1976, 32(9), 1041-3).
All the above-described processes are very difficult to practice on an industrial scale either due to the high costs of the equipment involved by the presence of F.sub.2, or due to the use of catalysts such as, for example, CoF.sub.3, which requires particularly sophisticated apparatus for the regeneration with F.sub.2, which is necessary due to the catalyst reduction during the reaction.
French patent application No. 86,08389 in the name of the Applicant hereof describes a process for preparing halogenated olefins by catalytic hydrogenation of 1,2-dichlorodifluoroethylene at temperatures higher than 100.degree. C., in particular from 200.degree. to 600.degree. C., preferably from 300.degree. C. to 400.degree. C.
According to the teaching of said patent application, the hydrogenolysis of 1,2-dichlorodifluoroethylene in the presence of palladium as a catalyst involves the partial or total substitution of chlorine, while the ethylenic unsaturation of the starting compound is left unaltered. All the experimental examples described in the cited French patent application were operated at a temperature of 300.degree. C. or 350.degree. C.
High conversions, even higher than 90%, were obtained and, as by-products, a mixture of CH.sub.2 F--CH.sub.2 F, CHClF--CHClF and CH.sub.2 .dbd.CHF, in an amount not exceeding 20%, was obtained.
According to this patent application, the molar ratio between hydrogen and olefin can vary over a wide range, from 0.5 to 10, preferably from 3 to 5. The lower the reaction temperature the greater is the necessity to use high ratios between hydrogen and olefins, namely higher than a ratio of 2:1, in order to obtain high yields of the olefins. The temperature being equal, high yields of the olefins are obtained if it is operated with high molar ratios, higher than 2:1.
From an older patent application filed by the Applicant (European patent application 253,410) it is known how to prepare fluorinated or chloroinated olefins, in particular fluoro- and chloroethylenes such as CHF.dbd.CHF and CFCl.dbd.CFH, starting from chlorofluoroethanes and hydrogen in the presence of hydrogenation catalysts.
The reaction temperatures range from 150.degree. to 600.degree. C., preferably from 200.degree. to 400.degree. C.
According to the process of said application, experimental tests have proved that it is necessary to operate at high temperatures and with high ratios between hydrogen and chlorofluoroethane in order to obtain high conversions.
However, tho conversion is never total, its maximum value being about 85%, which leads, in the course of time, to a deactivation of the catalyst and, by consequence, to serious drawbacks for a commercial-scale process.